New typhoid vaccines are urgently required. Salmonella serovars are rapidly developing resistance to antibiotics and licensed vaccines have safety concerns or are pooriy immunogenic. Individuals living in endemic areas often suffer from repeated bouts of typhoid and relapse of primary infection also occurs in 5- 15% of patients. Thus, Salmonella can persist in an immune competent host and acquired immune responses appear unable to completely eradicate infection. Understanding the basis of this immunological problem is critical for the development of effective vaccines against typhoid and is the primary focus of this sub-project. In the previous funding period, we developed a mouse model that allows study of persistent Salmonella shedding, antibiotic-mediated relapsing disease, and the failure to develop robust immunity to this bacterial infection. In this renewal application we propose to study this model in detail and determine the anatomical location of bacteria persistence and develop a detailed understanding of B cells and T cell responses in protective immunity. Our focus on adaptive immunity to enteric and persisting infections provides significant overiap with projects 1 (Lefrancois) and 4 (Cauley). The specific aims of this sub-project are: Aim 1. To examine a new model of relapsing Salmonella infection to test thel hypothesis that systemic spread of bacteria initiates from intestinal epithelial cells (lEC). Aim 2. To visualize Sa/mone//a-specific multifunctional Thi cells and define their role in protection against primary, secondary', and relapsing typhoid. Aim 3. To visualize Sa/mone//a-specific B cells in vivo and define the role of antibod^ in protection against relapsing infection. Our new preliminary data describe the recent development of state-of-the-art technology to track Sa//T)orje//a-specific lymphocyte responses in vivo, and the generation of ajnovel antibiotic-treatment model to study relapsing typhoid. Our hypothesis is that B and T cells play essential, non-overiapping, roles in bacterial clearance during primary and relapsing Salmonella infection. i RELEVANCE (See instructions): Typhoid kills over 200,000 people every year in developing countries and is recognized as a potential bioterrorist threat in the US. This proposal aims to understand why the adaptive immune response to Salmonella is unable to clear primary infection and allows for persistent and relapsing infection to occur. Understanding these unusual features of host-pathogen interaction will be crucial to the development of effective typhoid vaccines and will expand our understanding of persistent and relapsing bacterial infections.